A main tenet of both electrical and optical engineering is the desirability of filtering, sorting and processing information with higher degrees of precision. In electrical engineering, a significant breakthrough in precision filtering and signal processing came with the active filter. In electrical engineering, an active filter is one that includes an electronic gain element. In very early examples of the art, the electronic gain element was a vacuum tube. For the past half century, the electronic gain element has been a transistor. The improvement in filtering precision due to an electronic gain element may be intuitively understood by a simple band pass example. A passive electronic band pass filter may be made from a conductor, a capacitor, and a resistor, and will attenuate frequencies away from resonance more than frequencies near resonance. An active electronic band pass filter that includes a transistor will show improved performance because frequencies near resonance may now be amplified. Active low pass, high pass, matched, and other varieties of electronic filters also show improved performance over their passive counterparts.
Currently, there are extensive examples of passive optical filters that act to sort one frequency of light from another, separate bands of frequencies, or preferentially select a set of frequencies from another. For example, a diffraction grating will separate different colors of light into different propagation directions, allowing some to be spatially filtered. Diffraction gratings find wide applications in monochromators and spectraphotometers, as well as in dense wave division multiplexed (DWDM) telecommunications systems. For a second example, a thin film coating filter may be used to greatly reduce or greatly increase the reflected light from an air-glass interface. Anti-reflection (AR) thin film coatings find wide application in camera, telescope and eyeglass lenses. High reflectivity (HR) thin film coatings find wide application in laser mirrors. Thin film filters also find wide application in DWDM telecommunication systems to add, drop and otherwise sort channels.
A shortcoming of the optical filters currently known is that they are passive. Current optical filters do not have gain, and thus their performance is limited. For example, the quality factor of a filter is equal to a resonant frequency divided by the uncertainty in that frequency f/(Δf). It is well known that the quality factor of a passive filter is lower than the quality factor of an active filter of the same order. Thus, there is a need for an optical filter that is active and yields higher performance including higher quality factors. This will enhance tunabilty of such filters and provide numerous other benefits.
Another shortcoming of the optical filters currently known is that they are manufactured for specific applications. It would be very desirable to provide optical filters that could be readily constructed from combinations of conventional or standardized elements or components. Providing this capability could significantly enhance flexibility in designing optical filters and could also reduce costs associated therewith.